In recent years, a water absorbing resin has been in widespread use, as a main constituent material of a water absorbent core for absorbing body fluids, in sanitary materials such as a disposable diaper, a sanitary napkin, and an incontinence pad, for the purpose of absorbing body fluids such as urine and blood.
Known as the above water absorbing resin are such as: a partially neutralized and crosslinked polyacrylic acid; a hydrolysate of starch-acrylonitrile-grafted polymer; a neutralized starch-acrylic acid-grafted polymer; a saponified vinyl acetate-acrylic acid ester copolymer; a crosslinked carboxymethylcellulose; a hydrolysate of acrylonitrile copolymer or a hydrolysate of acrylamide copolymer, or a crosslinked acrylonitrile copolymer or a crosslinked acrylamide copolymer; a crosslinked cationic monomer; an isobutylene-maleic acid crosslinked copolymer; and a crosslinked polymer of 2-acrylamide-2-methylpropanesulfonic acid and acrylic acid.
Water absorbing characteristics that have been conventionally required for the water absorbing resin are such as: high absorption capacity when the water absorbing resin contacts an aqueous liquid such as body fluid; excellent absorption speed; liquid permeability; gel strength of a swelling gel; and an amount of water that the water absorbing resin absorbs from a base material containing the aqueous liquid.
In recent years, sanitary materials such as disposable diapers have been more functionalized and made thinner. In order to make the sanitary materials thinner while increasing the absorbing amount and preventing liquid leakage, more water absorbing resin have been used, and a larger ratio of the water absorbing resin to an entire water absorbent core in the sanitary material have been used. Such a water absorbent core including an increased amount of water absorbing resin is preferable when keeping liquid therein is merely considered. However, actual use of the water absorbent core in a sanitary material such as a disposable diaper is associated with such a problem that the water absorbing resin swells when absorbing water and turns into a soft gel state, and causes a gel blocking phenomenon in which any more fluids (urine or blood) cannot be absorbed into the water absorbent core, especially when human's body weight is bearing on the sanitary material such as a disposable diaper, that is, especially under a load. This would lead to a decrease in absorbing amount or leakage of the fluid.
Under this circumstance, the liquid permeability of the water absorbing resin has been spotlighted in recent years, and there have been many reports on sanitary materials such as disposable diapers including the water absorbing resin having higher liquid permeability (See Patent Literatures 1 to 3).
Further, in the water absorbing resin, in order to both suppress the gel blocking and realize a sufficient absorption amount, it is necessary to obtain a water absorbing resin that is excellent in a balance between an absorption capacity represented by a centrifugal retention capacity (CRC), for example, and liquid permeability represented by a saline flow conductivity (SFC), for example. However, they have such a relationship that an increase in one results in a decrease in the other. This makes it difficult to improve the relationship (balance) between them to a successful level. As means for attaining such an object, the following techniques have been known, for example.
Patent Literature 4 discloses a water absorbing resin that has been processed with a tri- or more-valent cation.
Patent Literature 5 discloses a technique with which an electrostatic or stereoscopic spacer is used with a water absorbing resin.
Patent Literature 6 discloses a particulate water absorbing agent containing: water absorbing resin particles produced by (i) cross-linking a monomer containing acrylic acid and/or salt thereof, and (ii) further cross-linking a surface of each of particles thus obtained, which particles have been pulverized into irregular shapes; and an agent for enhancing liquid permeability. An object of Patent Literature 6 is to provide a water absorbing agent having properties of both capillary suction pressure and liquid permeability.
Meanwhile, other than Patent Literature 4 through 6 whose object is to improve the balance between the absorption capacity and the liquid permeability, there has been proposed to mix various additives for the purpose of an improvement in various properties of a water absorbing resin.
For example, Patent Literature 7 discloses a method of producing a water absorbing agent, including the steps of: processing surface of water absorbing resin particles with a cross-linking agent, the water absorbing resin particles having a carboxyl group and the cross-linking agent having two or more functional groups that can form a covalent bond by reacting with the carboxyl group, so as to crosslink a part of the carboxyl group; and then mixing the particles with a cationic polymer compound having a weight-average molecular weight of 2000 or more, which cationic polymer compound can form an ion bond by reacting with the carboxyl group. Thereby, Patent Literature 7 easily obtains a water absorbing agent that hardly moves or is omitted in complex with a cellulose fiber, which water absorbing agent has such a feature so as to be expected to have a synergetic effect in absorption performance.
Further, Patent Literature 8 discloses a water absorbing agent composition containing water-insoluble inorganic powder fine particles and/or a polyamine compound having a weight-average molecular weight of 5000 or more, wherein: under a load of 20 g/cm2 (1.96 kPa), 0.9 weight % sodium chloride solution has a diffusing absorption capacity of 25 g/g or more after a lapse of 60 minutes from the beginning of absorption.
Furthermore, Patent Literature 9 discloses a water absorbing agent composition containing: water absorbing resin particles having at least an anionic dissociable group; and water-swelling resin particles having a cationic group. Thereby, Patent Literature 9 provides a water absorbing agent composition that is more excellent in an absorption speed, a permeability rate of a gel layer, a permeability rate of a gel layer under pressure, and an absorption capacity against pressure respectively, than each of the two kinds of the resin particles solely used.
Patent Literature 10 discloses water absorbing resin particles, wherein: at least a part of a surface of the water absorbing resin particles is coated with a coating film that is made from an adduct of a polyvalent amine compound and a compound having an olefinic double bond. Thereby, Patent Literature 10 provides: a water absorbing agent that is advantageous in an absorption capacity under pressure or no pressure, provides high safety for a human skin and the like, and is suitable for use in a sanitary material; and a method of producing the water absorbing agent.
Further, Patent Literature 11 discloses a mixture of water absorbing resin particles and a cationic polymer compound in which a specific region is crosslinked. Thereby, Patent Literature 11 realizes that, even long after a swelling, it is possible to (i) keep a state where gels clump together and retain a shape, and (ii) have high liquid permeability and a high absorption capacity against pressure.
Patent Literature 12 discloses a water absorbing agent made from a nitrogen-containing polymer having a nitrogen atom that can realize a protonation of 5 to 17 mole/kg against a total mass of particles of a water absorbing agent polymer, and the nitrogen-containing polymer. Thereby, Patent Literature 12 provides a water absorbing agent having: an improved property of high absorption performance; improved liquid permeability; and high water resistance.
Patent Literature 13 discloses a water absorbing resin produced by compounding an acid crosslinked polymer and a basic crosslinked polymer. In Patent Literature 13, a crosslinked polyethylenimine, and a crosslinked polyallylamine are employed as the basic crosslinked polymer.
Meanwhile, there have been many reported water absorbing agents that are crosslinked or coated with polyamine, as shown in Patent Literatures 14 through 31. Among those, there has been reported a water absorbing agent, in which a condensate of (i) one of animes and ammonia and (ii) epihalohydrin is used as a cross-linking agent (Patent Literatures 30 and 31). Patent Literature 30 discloses a method for modifying a resin with high water absorbability. In this method, a water-insoluble resin with high water absorbability having a —COOM group is crosslinked with a reactant of (i) the epihalohydrin and (ii) one of the ammonia and the amines, in order to increase the absorption capacity under pressure, without reductions in the gel strength and absorption capacity without pressure.
Further, Patent Literature 31 discloses that in producing the resin with high water absorbability by polymerizing a polymerizable monomer having a partly-neutralized carboxyl group, if the reactant of (i) the epihalohydrin and (ii) one of the ammonium and the amines is added as the cross-linking agent, a polymerization reaction proceeds smoothly and thereby the crosslinking can be evenly preformed.
Furthermore, Patent Literatures 5, 11, 12, and 25 disclose a method for producing the water absorbing agent, in which the water absorbing resin and the cationic polymer are mixed. A lot of examples of the cationic polymers disclose the condensate of the amines and the epihalohydrin.
The absorption capacity and the liquid permeability generally conflict, and improvement in the liquid permeability has been generally associated with decrease in the absorption capacity. Further, the techniques disclosed in Patent Literatures 4 through 6 are techniques for accomplishing an improvement in relationship between the absorption capacity and the liquid permeability (for realizing a water absorbing resin having higher liquid permeability compared to that having the same absorption capacity) by adding an additive having an effect of increasing the liquid permeability, such additive as polyvalent metal salt (metal cation), inorganic particles, or a polycation, to a water absorbing resin. However, the relationship (balance/improvement in liquid permeability) has still not reached a sufficient level, and much improvement has been demanded. Furthermore, the techniques disclosed in Patent Literatures 7 through 31 have not taken the relationship (balance/improvement in liquid permeability) between the absorption capacity and the liquid permeability to the sufficient level.
Specifically, it has been demanded for the water absorbing agent to have both a high centrifugal retention capacity (CRC) and a high saline flow conductivity (SFC) when the water absorbing agent is actually used. However, the conventional techniques have not successfully satisfied these properties.